Quick Answer
A tire size like P255/45R19 104W decodes as: P = Passenger (TRA standard), 255 = section width in mm, 45 = sidewall height is 45% of 255 mm (114.75 mm tall), R = Radial construction, 19 = fits a 19-inch rim, 104 = load index (900 kg / 1,985 lb per tire), W = speed rating (270 km/h / 168 mph maximum).
Overall diameter: (2 × 114.75) + (19 × 25.4) = 229.5 + 482.6 = 712.1 mm (28.04 inches).
What Common Tire Codes Mean: 91W, 94W XL, 96W XL, 104W, P255
The last part of a tire size is the service description: a load index number, a speed rating letter, and sometimes XL. These are the codes drivers look up most often.
| Code | Load index | Speed rating | Notes |
|---|---|---|---|
| 91W | 91 = 615 kg (1,360 lb) per tire | W = 270 km/h (168 mph) | Standard load (SL), the usual pairing for 225/45R17 |
| 94W XL | 94 = 670 kg (1,480 lb) per tire | W = 270 km/h (168 mph) | XL means Extra Load: reinforced, rated for a higher inflation pressure |
| 96W XL | 96 = 710 kg (1,570 lb) per tire | W = 270 km/h (168 mph) | Extra Load tire for heavier cars, same outside dimensions as a standard load tire |
| 104W | 104 = 900 kg (1,980 lb) per tire | W = 270 km/h (168 mph) | Common on SUVs and trucks such as P255/45R19 104W |
| P255 | P = passenger tire (TRA standard), 255 = section width in millimeters (about 10 inches) | A tire without the P is Euro-metric, with the same geometry | |
Load index values follow the standard table further down this page; Michelin, for example, lists load index 91 as 615 kg and explains XL as Extra Load, a reinforced tire. For a number that is not listed here, use the tire load index calculator.
Every Number on Your Tire Sidewall, Explained
The sidewall of every tire contains 7–8 pieces of standardized information. Source: TireCalculatorHub.com
P255/45R19 104W: Segment by Segment
| Segment | Name | Meaning |
|---|---|---|
| P | Service type prefix | Michelin: P is passenger car, LT is light truck, C is commercial van and T is a temporary spare. No prefix is the Euro-metric marking. |
| 255 | Section width | Tire width in millimeters from sidewall to sidewall at the widest point. |
| 45 | Aspect ratio | Sidewall height as a percentage of section width. 45 means the sidewall is 45% of 255 mm = 114.75 mm tall. |
| R | Construction type | R = Radial (cord plies at ~90° to the direction of travel). Nearly all modern passenger and LT tires are radial. |
| 19 | Rim diameter | Wheel diameter the tire fits, in inches. |
| 104 | Load index | Numeric code for maximum load per tire at rated pressure. Load index 104 = 900 kg (1,985 lb) per tire. |
| W | Speed rating | Maximum sustained speed capability. W = 270 km/h (168 mph). |
Service Type Prefix: P, LT, ST, T, or None
The optional letter (or letters) before the section width tells you what category of service the tire is designed for and which standards organization set its load tables.
| Prefix | Standard | Vehicles | Key Note |
|---|---|---|---|
| P | TRA (USA) | Cars, minivans, crossovers, most light SUVs | P stands for passenger car tires; check the load index and speed rating on your placard |
| LT | TRA (USA) | ¾-ton and 1-ton pickups, commercial vans, heavy tow vehicles | LT stands for light truck tires; LT tires carry a load range letter (for example E = 10-ply rating, Continental and Carlisle) |
| ST | TRA (USA) | Trailer axles only | ST stands for Special Trailer (Carlisle). Carlisle lists ST speed symbol J = 65 mph and recommends towing at no more than 60 mph |
| T | TRA (USA) | Temporary spare ("space saver") | Vehicle manuals (Toyota, Lexus, Mazda, Jeep, RAM, Audi) limit a compact spare to 50 mph |
| (none) | ETRTO (Europe) | European imports and many global fitments | Same size geometry as P-metric; check the load index and speed rating on your placard |
Euro-metric vs P-metric
A tire marked 225/60R16 98H (no prefix) is Euro-metric. A tire marked P225/60R16 97His P-metric. Check the load index and speed rating on your placard; Michelin says a tire's load rating must always meet or exceed the vehicle manufacturer's value, and to replace tires with the same size designation or approved options.
Geometry Formulas and Worked Example
Worked Example: 225/45R17 96W
Section width: 225 mm
Aspect ratio: 45 → Sidewall = 225 × 0.45 = 101.25 mm (3.99 in)
Rim diameter: 17 in → 17 × 25.4 = 431.8 mm
Overall diameter: (2 × 101.25) + 431.8 = 634.3 mm (24.97 in)
Circumference: π × 24.97 ≈ 78.5 in
Load index 96: 710 kg (1,570 lb) per tire maximum
Speed rating W: 270 km/h (168 mph) maximum
Summary: ~225 mm wide, ~4 in sidewall, ~25 in total diameter, rated to carry up to 710 kg at sustained speeds up to 168 mph when correctly inflated.
XL (Extra Load) vs SL (Standard Load)
XL is often misread as "extra large": it means Extra Load. Michelin says XL tires have a reinforced internal structure that lets them carry more weight at higher inflation pressures than standard load (SL) tires of the same size. The external dimensions are identical: only the maximum load capacity and recommended pressures change. Always check the door jamb sticker: if your vehicle came with XL tires, replacements must also be XL.
Speed Rating Table: L through ZR
| Symbol | Max km/h | Max mph | Typical Use |
|---|---|---|---|
| L | 120 | 75 | Off-road and light truck tires (Michelin chart) |
| M | 130 | 81 | Temporary spare tire (Michelin chart) |
| N | 140 | 87 | Temporary spare tires (Michelin chart) |
| P | 150 | 93 | Not given in Michelin chart |
| Q | 160 | 99 | Winter 4x4 (Michelin chart) |
| R | 170 | 106 | Heavy duty light truck (Michelin chart) |
| S | 180 | 112 | Family sedans and vans (Michelin chart) |
| T | 190 | 118 | Family sedans and vans (Michelin chart) |
| U | 200 | 124 | Sedans and coupes (Michelin chart) |
| H | 210 | 130 | Sport sedans and coupes (Michelin chart) |
| V | 240 | 149 | Sport cars (Michelin chart) |
| W | 270 | 168 | Exotic sport cars (Michelin chart) |
| Y | 300 | 186 | Exotic sport cars (Michelin chart) |
Michelin says replacement tires should meet or exceed the speed rating specified by the vehicle manufacturer, and that a speed rating is the maximum speed at which a tire can carry its maximum load under specified conditions. Bridgestone says it is not a recommended travel speed. To look up one letter in mph and km/h, the speed rating lookup does it in one step.
Load Index Reference Table (70–120)
The load index corresponds to the maximum weight a single tire can support when properly inflated (Michelin), from the International Load Index table.
| Load Index | kg | lb |
|---|---|---|
| 70 | 335 | 739 |
| 71 | 345 | 761 |
| 72 | 355 | 783 |
| 73 | 365 | 805 |
| 74 | 375 | 827 |
| 75 | 387 | 853 |
| 76 | 400 | 882 |
| 77 | 412 | 908 |
| 78 | 425 | 937 |
| 79 | 437 | 963 |
| 80 | 450 | 992 |
| 81 | 462 | 1,019 |
| 82 | 475 | 1,047 |
| 83 | 487 | 1,074 |
| 84 | 500 | 1,102 |
| 85 | 515 | 1,135 |
| 86 | 530 | 1,168 |
| 87 | 545 | 1,201 |
| 88 | 560 | 1,235 |
| 89 | 580 | 1,279 |
| 90 | 600 | 1,325 |
| 91 | 615 | 1,355 |
| 92 | 630 | 1,390 |
| 93 | 650 | 1,435 |
| 94 | 670 | 1,475 |
| 95 | 690 | 1,520 |
| 96 | 710 | 1,565 |
| 97 | 730 | 1,610 |
| 98 | 750 | 1,655 |
| 99 | 775 | 1,710 |
| 100 | 800 | 1,765 |
| 101 | 825 | 1,820 |
| 102 | 850 | 1,875 |
| 103 | 875 | 1,930 |
| 104 | 900 | 1,985 |
| 105 | 925 | 2,040 |
| 106 | 950 | 2,095 |
| 107 | 975 | 2,150 |
| 108 | 1,000 | 2,205 |
| 109 | 1,030 | 2,270 |
| 110 | 1,060 | 2,335 |
| 111 | 1,090 | 2,405 |
| 112 | 1,120 | 2,470 |
| 113 | 1,150 | 2,535 |
| 114 | 1,180 | 2,600 |
| 115 | 1,215 | 2,680 |
| 116 | 1,250 | 2,755 |
| 117 | 1,285 | 2,835 |
| 118 | 1,320 | 2,910 |
| 119 | 1,360 | 3,000 |
| 120 | 1,400 | 3,085 |
★ Common passenger car load indexes. Scroll to see all 51 values.
Flotation Tire Sizes: 33x12.50R15LT Decoded
Flotation (inch) sizes are common on off-road trucks and lifted SUVs. To see one written in metric, check the flotation to metric conversion table; for the metric format itself, the guide to reading metric tire sizes goes through each part. The format is: Overall Diameter × Section Width R Rim Diameter Construction
Overall diameter
33 inches tall (nominal unloaded height)
Section width
12.5 inches wide (= 317.5 mm)
Construction
Radial
Rim diameter
15-inch wheel
Service type
Light Truck construction (same LT rules apply)
Converting 33x12.50R15 to P-Metric Equivalent
Section width: 12.50 in × 25.4 = 317.5 mm
Sidewall height: (33 − 15) ÷ 2 = 9 in × 25.4 = 228.6 mm
Aspect ratio equivalent: 228.6 ÷ 317.5 ≈ 72%
The nearest real catalog P-metric size is 315/75R15: which verifies to ≈ 33 inches overall diameter. Note that flotation diameters are nominal; actual mounted diameter can vary 0.5–1 inch by brand and rim width.
When are flotation and P-metric sizes NOT interchangeable?
- × A 12.50-inch flotation tire may be too wide for the OE rim and rub suspension or body where a 245 or 255 P-metric did not
- × Flotation LT tires often have Load Range C/D/E with stiffer construction than OE P-metric SL tires
- × Taller flotation tires (33–35 in) can contact fender liners or control arms at full suspension compression even when the nominal diameter seems close on paper
OEM Tire Sizes Named in Owner's Manuals
| Category | Width (mm) | Aspect Ratio | Rim (in) |
|---|---|---|---|
| Lexus ES (2024 manual) | 215–235 | 40–55 | 17–19 |
| Toyota Tacoma (2025 manual) | 245–265 | 65–70 | 17–18 |
| Ford F-150 (manuals) | 245–275 | 50–70 | 17–22 |
| RAM 2500 (2025 manual) | 245–285 | 60–70 | 17–20 |
Ranges are taken from the sizes in each row. Your tire placard lists the sizes for your vehicle.
Changing Tire Size: The 3% Rule and Its Limits
Speedometer Error Formula
Example: Replacing 225/60R16 (26.64 in) with 245/65R17 (29.54 in): Ratio = 29.54 ÷ 26.64 = 1.109 → ~11% error. At 60 mph indicated, you are actually travelling ~66.5 mph. At 70 mph indicated, actual speed is ~77.6 mph.
| System Affected | What Goes Wrong Beyond 3% |
|---|---|
| Speedometer / Odometer | A larger diameter makes the speedometer read lower than actual speed: actual speed = (new diameter ÷ original diameter) × indicated speed (calculated). |
| Load capacity and speed rating | Michelin: replacement tires must meet or exceed the vehicle manufacturer's load rating and speed rating. |
| Vehicle approval | Michelin: replace tires with the same size designation, or approved options, recommended by the vehicle manufacturer. Ford's manuals say tires and wheels must be the same size, load index and speed rating as those originally fitted. |
| All-wheel drive | Michelin: without instructions in the owner's manual, all four tires must be the same size and within 1 inch (2.5 cm) of each other in outside circumference. |
Plus-Sizing: How to Calculate the Correct Aspect Ratio
To go from 17-inch to 18-inch wheels while keeping overall diameter the same:
Example: OE is 225/50R17 → Overall diameter = (2 × 112.5) + 431.8 = 656.8 mm. Going to 18-inch wheel with 235 mm width: H_new = (656.8 − 457.2) ÷ 2 = 99.8 mm. AR_new = 99.8 ÷ 235 ≈ 42.5% → nearest catalog size is 235/40R18 (≈1.8% smaller) or 235/45R18 (≈1.8% larger). Both within the 3% rule.
The 3% Rule Is Not the Whole Picture
Staying within 3% diameter is necessary but not sufficient. You must also maintain or exceed OEM load index, OEM speed rating, adequate clearance at full suspension compression and full steering lock, and all four tires must be the same size (unless OEM specifies staggered fitment). A 2% diameter change with a lower load index can be far more dangerous than a 4% change with a higher load index on a properly recalibrated vehicle.
Frequently Asked Questions
What does P mean in a tire size like P225/45R17?
P stands for passenger car tires, and LT stands for light truck tires. A tire without a prefix (for example 225/45R17) is Euro-metric. Check the load index and speed rating on your placard rather than relying on the prefix; Michelin says a tire's load rating must always meet or exceed the vehicle manufacturer's value.
What is the aspect ratio on a tire?
The aspect ratio is the second number in a tire size: it describes the sidewall height as a percentage of the section width. In 225/45R17, the aspect ratio is 45, meaning the sidewall height is 45% of 225 mm = 101.25 mm (about 4 inches). A lower aspect ratio means a shorter sidewall (low-profile tire), which improves steering response and reduces sidewall flex but reduces ride cushioning. A higher aspect ratio means more sidewall height, more cushioning, and better off-road compliance.
How do I calculate overall tire diameter?
Use these formulas: Sidewall height (mm) = Section Width × (Aspect Ratio ÷ 100). Overall diameter (mm) = (2 × Sidewall Height) + (Rim Diameter in inches × 25.4). Convert to inches: divide by 25.4. Example: 225/45R17 → Sidewall = 225 × 0.45 = 101.25 mm → Diameter = (2 × 101.25) + (17 × 25.4) = 202.5 + 431.8 = 634.3 mm = 24.97 inches.
What does XL mean on a tire?
XL stands for Extra Load. Michelin says XL tires have a reinforced internal structure that lets them carry more weight at higher inflation pressures than standard load (SL) tires of the same size. If your vehicle's factory tires are XL, replacements must also be XL (Michelin). The external dimensions of the same size are the same.
Can I use Euro-metric tires instead of P-metric?
Check your owner's manual or ask the vehicle manufacturer. Michelin says to replace tires with the same size designation, or approved options, recommended by the vehicle manufacturer, with a load rating and speed rating that meet or exceed the specified values.
What is the 3% tire size rule?
The 3% figure is from Continental, which advises keeping rolling circumference within 3% of the original; it is not an NHTSA regulation or a vehicle maker specification. Ask your vehicle manufacturer which sizes are approved, and use the calculators on this site to see the exact percentage and the speedometer error.
What is a flotation tire size like 33x12.50R15?
Flotation sizes (also called inch sizes) are used for off-road and light truck tires. The format is overall diameter × section width R rim diameter. In 33x12.50R15LT: 33 = 33 inches tall, 12.50 = 12.5 inches wide, R = radial, 15 = 15-inch rim, LT = light truck. Its closest metric size is calculated as roughly 315/70R15 (width 12.5 × 25.4 = 317.5 mm; sidewall (33 − 15) ÷ 2 = 9 inches = 228.6 mm, about 72% of the width).
How does changing tire size affect speedometer readings?
Speedometers count wheel revolutions and multiply by the expected circumference. If you install tires with a larger diameter, each revolution covers more ground than the vehicle expects: so the speedometer reads lower than your actual speed. Formula: Actual Speed = (New Diameter ÷ Original Diameter) × Indicated Speed. A 10% larger diameter means you are actually traveling 10% faster than the speedometer shows. At 60 mph indicated, you are actually doing 66 mph.
What is plus sizing?
Plus sizing means increasing wheel diameter while reducing the tire's aspect ratio to keep overall diameter approximately the same. Example (calculated): 225/50R17 to 235/40R18 changes the overall diameter by about 1.8%. Ask the vehicle manufacturer whether a size is approved. To calculate the aspect ratio for a new rim: H_new = (Original Diameter mm − (New Rim in inches × 25.4)) ÷ 2.
What load index should my tires have?
Your replacement tires must meet or exceed the load index specified for your vehicle, found on the tire placard (Michelin). A higher load index is permitted but does not raise the vehicle's maximum allowable load. For vehicles originally fitted with LT tires, follow the load range and load index on the placard.
Free Tire Size Calculators
Universal Tire Converter
Convert between P-metric, flotation, and inch sizes with full dimension breakdown
Tire Size Comparison
Compare two tire sizes side by side: diameter, width, sidewall, speedometer error
Speedometer Error Calculator
Calculate how much your speedometer reads off after a size change
Plus Sizing Calculator
Find compatible sizes when upsizing to a larger rim diameter
Load Index Calculator
Look up maximum load in kg and lb for any load index number
Overall Tire Height
Calculate exact overall diameter and sidewall height for any tire size
Disclaimer: All tools and information provided on TireCalculatorHub are for informational purposes only. Always consult with a qualified professional before making decisions about your vehicle's tires or modifications. TireCalculatorHub is not liable for any damages or issues arising from the use of information or tools on this website.